The United Nations began bringing African refugees to Italy from Libya on Friday, evacuating them from detention centres where conditions have been condemned by relief groups as inhumane.

Hundreds of thousands of migrants have fled conflict or poverty in their home countries and are now trapped in Libya, where they had hoped to pay smugglers for passage to Europe via Italy.

It is the first time the UN High Commissioner for Refugees (UNHCR) in Libya has evacuated refugees directly to Europe. An Italian C-130 military plane landed at an airport south of the capital carrying 110 women and children, and a second flight is expected to bring more than 50 people later in the day.

The UNHCR classifies the arrivals as "vulnerable" refugees, which means they are children, victims of abuse, women, the elderly or have disabilities.

The African migrants were covered in blankets or bundled in coats as they disembarked from the plane on a chilly evening.

"We really hope other countries will follow the same path," Vincent Cochetel, UNHCR's Special Envoy for the Central Mediterranean, said in a statement after the first plane arrived.

The UNHCR estimates about 18,000 people are being held in detention centres for immigrants that are controlled by the Tripoli government and it aims to evacuate as many as 10,000 next year.

Italy's Catholic church will house many of the new arrivals in shelters across the country, church charity Caritas said, as the migrants from Eritrea, Ethiopia, Somalia and Yemen go through the country's asylum request process.

Migrant arrivals to Italy have fallen by two-thirds since July from the same period last year after officials working for the UN-backed government in Tripoli persuaded human smugglers in the city of Sabratha to stop boats leaving.

In August, MEE revealed that armed groups are reportedly receiving aid from Italy to stop the boats leaving Libya.

Italy is also bolstering the Libyan coastguard's ability to turn back boats.

Italy's move to open a safe corridor for some of the migrants follows criticism by rights groups that have condemned the country's efforts to block migrants in Libya in exchange for aid, training and equipment to fight smuggling.

"This should be a point of pride for Italians," Interior Minister Marco Minniti told reporters on the tarmac. "This is the just beginning. We will continue to try to open this humanitarian corridor."

Reports of slavery and human trafficking against migrants in Libya had sparked international outrage.

Earlier this month, Amnesty International accused European governments of being partly responsible for rights violations against migrants in Libya.

“European governments have not just been fully aware of these abuses; by actively supporting the Libyan authorities in stopping sea crossings and containing people in Libya, they are complicit in these crimes,” Amnesty’s Europe Director, John Dalhuisen, said in a statement.

Tens of thousands of migrants are estimated to be detained by smugglers, and the African Union says that as many as 700,000 migrants are in Libya. The UNHCR has registered more than 44,000 as refugees and asylum seekers.